Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
FOOD FREEZING AND THAWING METHOD AND APPARATUS
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] This invention generally pertains to systems and methods for freezing
and
for thawing food. More parfiicularly, the present invention is directed to
systems and
methods of freezing food products that minimize damage to the food, such as
aging,
that may occur during the freezing process. The present invention also relates
to
systems and methods for thawing frozen foods to maximize taste.
Description of the Related Art
[0002] In conventional prior art freezing methods, food is reduced in
temperature from room temperature to the frozen state in a matter of hours,
typically
1 to 3 hours. When such conventional methods are applied to high water content
foods such as sushi (which is a well known combination of cooked rice, raw
fish and
other toppings), a substantial portion of the water in'the food is
irreversibly lost. The
loss of water is caused by an accelerated aging process that takes place when
the
food is exposed to a certain temperature zone for a relatively long period of
time
during conventional freezing processes. Exposure to this accelerated aging
temperature zone for prolonged periods of time also results in the generation
of ice
crystals at a high rate. As a result, ice crystals that form will expand in
size with time
and rupture the cell structure of the food being frozen. When the food is
defrosted,
water generated from the ice crystals will be irreversibly lost from the food.
Thus,
conventional prior art food freezing methods have substanfiial drawbacks
resulting
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from the substantial loss of moisture content, cell structure damage, thereby
reducing freshness and changing the texture and desirability of the thawed
food
product.
[0003] In connection with efforts to improve conventional prior art freezing
methods, many professional and industrial "quick" freezer systems use low
temperature nitrogen gas or carbon dioxide gas as a cooling medium for more
rapid
(flash) freezing purposes. UVhile nitrogen gas has a low temperature
capability
(-196°C), its specific heat is only about 47 Kcal/gram/°C, and
therefore is not
sufficient in terms of heat absorption capacity to extract heat from the bulk
of the
food at high rates. While conventional freezers create fractured food cells
due to ice
crystal growth, quick freezer systems utilizing low calorie cooling sources
may
damage food cells due to rapid freezing of the food. In both cases, food cells
are
destroyed during the freezing process. Carbon dioxide gas has a higher
specific
heat than nitrogen gas (about 137 Kcal/gram/°C), but has a much higher
minimum
temperature (about -79°C). Quick freezing systems using carbon dioxide
gas
encounter the same problems with high water content foods as described above.
[0004] In another attempt to address. shortcomings with conventional freezing
techniques, it has been proposed to apply a magnetic field to the food during
the
freezing process. In this approach, according to U.S. Patent No. 6,250,087,
magnetic energy is applied to the food to be frozen in a conventional freezer
to
attempt to prevent cell fracture caused by ice crystal growth during the
freezing
process. The food is shaken by the application of the magnetic field to
suppress
crystallization. However, this approach uses conventional freezing technology
and
the process still takes a long time for complete freezing to take place (2 to
3 hours),
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While it is asserted that this approach maintains moisture in the cell and
prevents
dripping, such systems are complex, expensive, and have limited capacity.
[0005] For the foregoing reasons, there is a need for new and improved systems
and methods for freezing and thawing food. The present invention overcomes
these
and other problems that occur with convention freezing techniques, and
particularly
in connection with freezing of higher water content foods.
SUMMARY OF THE INVENTION
[0006] In accordance with the foregoing and other objects, the present
invention
provides a method of freezing food for later thawing and use. The method
includes
the steps of packing a food product in a container for freezing, cooling the
food .
product substantially throughout the bulk thereof to about 10°C, and
then cooling the
food product substantially throughout the bulk thereof from about 10°C
to about 0°C
in less than approximately ten minutes.
[0007] According to another embodiment of the present invention, a method of
freezing a food product is provided which includes a step of packaging a food
product to be frozen after the temperature of the food product reaches a first
predetermined temperature. The food product is then cooled until the
temperature of
the food product reaches a second predetermined temperature. The food product
is
then cooled so that the temperature of the food product decreases from the
second
predetermined temperature to a third predetermined temperature within a first
predetermined period of time.
[0008] According to another embodiment of the present invention, a system for
freezing a food product is provided which comprises a freezer and a control
unit.
The freezer maintains an interior temperature set to a first temperature and
includes
a first cooling unit and an adjustable cooling unit providing additional
cooling energy.
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The control unit is coupled with the adjustable cooling unit and configured to
adjust
the additional cooling energy. The adjustable cooling unit provides additional
cooling
energy on demand.
[0009] According to the present invention, the calorie exchange rate of the
freezer
is adjusted to obtain the optimal freezing process to maintain the original
taste and
texture of the food. High water content foods, such as rice, can be frozen in
a short
period of time and in a manner that captures water in a food cell before large
ice
crystal clusters form and grow.
[0010] According to one embodiment of the present invention, dry ice is used
as a
cooling source in a double freezer configuration. When dry ice changes from
its
solid state to gas phase directly, a much higher calorie exchange rate.is
produced
than when liquid carbon dioxide changes phase to gas. The present invention is
a
simple, low cost system suitable to freeze a large capacity of food. Also, the
simple
design of the present invention includes a continuous frozen food chamber that
enables almost unlimited production of frozen foods.
[0011] According to another embodiment of the present invention, a method of
thawing frozen food is provided which comprises the steps of placing a
container of
coolant on a side of the frozen 'food, and steaming the frozen food from a
side that is
opposite to the side where the container of coolant is placed. The food is
steamed
until the food is thawed to a desired temperature.
[0012] According to the present invention, food is preferably frozen in a
reasonably short period of time fo avoid exposing the food to the maximum ice
crystal generation zone for extended periods of time which will cause damaging
food
by ice crystal growth. This is accomplished by using a, high calorie cooling
source,
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such as, for example dry ice. The freezing process of the present invention
avoids
the dehydration phenomenon resulting from conventional, quick freezing
methods.
[0013] According to the present invention, a method is provided for thawing
frozen food which includes a step of arranging a plurality of containers of
frozen food
in a tray. A package of coolant is placed on a side of each of said frozen
food. A
source of warm water is supplied to the tray until the plurality of containers
of frozen
food is thawed to a desired temperature.
[0014] With these and other objects, advantages and features of the invention
that may become hereinafter apparent, the nature of the invention may be more
clearly understood by reference to the following detailed description of the
invention,
the appended claims, and the drawings attached hereto.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] The invention will be described in detail with reference to the
following
drawings, in which like features are represented by common reference numbers
and
in which:
[0016] Fig. 1A is a block diagram of a.system for freezing food according to
an
embodiment of the present invention;
[0017] Fig. 1 B is a block diagram of a system for freezing food according to
another embodiment of the present invention;
[0018] Fig. 2A - 2B are side and top views of a tunnel type freezer according
to
another embodiment of the present invention;
[0019] Fig. 2C is a cross sectional partial side view of a tunnel type freezer
according to the embodiment in Figs. 2A and 2B;
[0020] Fig. 3 is a diagram showing a number of temperature sensors within the
interior freezer;
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[0021] Fig. 4 is a chart showing temperature versus time curves for freezing
or
thawing food;
[0022] Fig. 5 is a flow diagram of a method for freezing food according to an
embodiment of the present invention;
[0023] Fig. 6 is a diagram of a system for thawing food according to an
embodiment of the present invention;
[0024] Figs. 7A and 7B are illustrations of containers used in connection with
the
system for thawing foods according to the system of Fig. 6; and
[0025] Figs. 8A - 8C are illustrations of a system for thawing a large volume
of
containers of frozen foods according to an embodiment of the present
invention.
[0026] Figs. 9 is an illustration of a system for thawing a large volume of
containers of frozen foods according to another embodiment of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0027] Although the present invention is applicable to the freezing, and
thawing of
foods, and particularly those foods having high moisture content, the present
invention will be described in connection with a preferred embodiment directed
to
freezing and thawing the food product sushi.
[0028] In accordance with the present invention, sushi refers to any food
product
known as sushi such as, for example, a food product in the form of cooked rice
with
some form of topping (e.g., fish, avocado, etc.). Sushi can also be in the
form of
rolls. Sushi typically has a moisture content of about 60% by weight. There
are
several important factors to be considered when freezing high water content
food
which is intended to be defrosted later for consumption. One factor is the
aging
process by which foods like rice can irreversibly lose their water content. In
fihe case
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of sushi, this is a process by which a molecular chain of starch loses its
regular array
and turns into paste. The aging process in sushi is accelerated when the food
is
reduced to approximately below 10°C and is most severe through a
temperature
range of about 6°C to about 0°C. This temperature zone is
referred to as the
"accelerated aging temperature zone."
[0029] . A second factor is referred to as "the maximum ice crystal generation
zone," during which the water within the food forms into ice crystals. This
occurs, in
the case of sushi, in the range of from approximately 0°C down to
approximately -4
to -10°C. In this temperature zone, approximately 75% or more of the
water in the
food is transformed into ice crystals. The ice crystals damage the food during
formulation by destroying cell structure, drying, etc. The present invention
controls
the freezing process to ensure that food is passed through those two
temperature
zones in the desired time, but also ensures that the freezing occurs
throughout the
bulk of the food as well.
[0030] Fig. 1A is a block diagram of a food freezing apparatus according to an
embodiment of the present invention. Freezing apparatus 100 includes a first
freezer 102, a control unit 104, and a second freezer 106 contained within the
interior of first freezer 102. The first and second freezers may be any
commercially
available freezers which are capable of performing in accordance with this
disclosure
and are not meant to be limited except as expressly provided herein.
[0031] Second freezer 106 includes one or more cooling units 108 which
comprise a high calorie cooling source such as, for example, dry ice blocks.
Dry ice
may be provided in racks, as shown in Fig. 1 B. The second freezer 106 further
includes one or more variable cooling source discharge nozzles 112a which, in
a
preferred embodiment, discharge liquid C02 as a cooling source. Variable
cooling
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source nozzles 112a are preferably connected to a variable cooling source
112b,
which is connected to control unit 104. The second freezer 106 also preferably
includes one or more air circulation units or mechanisms 116, such as fans,
for
circulating the air within the second freezer thereby causing cooling by
convection as
well as conduction.
[0032] The system 100 may also include one or more cooling unit adjustment
mechanisms 110 that adjust the cooling units 108 to provide more or less heat
transfer (cooling) energy to the food 114 as needed depending on the size of
the dry
ice cluster and the volume of the food in the freezer. In one embodiment, the
cooling
adjustment mechanism is a rod or bar which is connected to each of the cooling
units 108 so that those units can be moved or rotated in unison. For example,
if
cooling units 108 include dry ice blocks, then the adjustment unit 110 is
preferably
used to change the angle of the blocks relative to the circulation units 116
to
increase or decrease heat transfer from the dry ice blocks to the food 114 by
providing more or less surface area of dry ice in contact with circulating
air. The
adjustment mechanism 110 can be used in connection with the manual adjustment
of the cooling units .108. In another embodiment, adjustment mechanism 110 can
be
used in connection with an automated adjustment of the cooling units 108. In
this
embodiment, electronic movement of the adjustment mechanism and cooling units
is
controlled by the control unit 104.
[0033] The dual-freezer configuration of the present invention provides a very
stable reference cooling temperature in the interior freezer 106. One skilled
in the
art will understand that single freezer arrangements can also be used. In
single
freezer arrangements, various loading systems may be used to prevent loss in
cooling energy during loading and unloading of food to be frozen, in order to
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maintain a steady interior temperature of the freezer. For example, suitable
loading
systems could include a loading chamber unit attached to a freezer with a door
on
the loading side and another door on the freezer side with an air tight seal.
During
the loading process, a door on the loading side is open, but the door on the
freezer
side remains closed. Once the food rack is loaded into the loading chamber, a
door
on the loading side is closed first and then the door on the freezer side is
open to
allow the food rack to enter inside of the freezer. When the food is
completely frozen
as described in the detailed description of the invention, the food rack is
preferably
taken out in the reverse order as described in connection with the loading
process.
[0034] The thermal exchange with the food to be frozen can be performed
smoothly using a high calorie cooling unit, such as dry ice, which has a very
high
calorie heat transfer coefficient. Food placed inside the second freezer 106
can
have its temperature passed through the accelerated aging temperature zone and
maximum ice crystal generation zone within a short period time by using a high
calorie cooling source.
[0035] The control unit 104 is coupled to the adjustment unit 110, variable
cooling
source 112b and circulation means 116, as well as to one or more temperature
sensors 118 which measure the temperature of the interior of freezer 106
and/or of
the food 114. The control unit 104 may include a computer processor or the
like, a ,
memory unit and appropriate input / output devices (not shown) for
communicating
with and controlling adjustment unit 110, variable cooling source 112b and
circulation
means 116, and for receiving temperature data from the one or more temperature
sensors 118. The control unit is preferably programmed with computer software
for
facilitating the processes of the present invention, which are described in
more detail
below.
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[0036] Fig. 1 B is a block diagram of freezing apparatus 200 according to
another
embodiment of the present invention. As shown, freezing apparatus 200 contains
a
freezer 206. Freezer 206 preferably contains one or more cooling units 108.
Cooling units 108 preferably are racks containing a cooling source such as,
for
example, dry ice blocks. One or more fans 116 are disposed along the walls of
freezer 206 in position to circulate air over the dry ice racks 108 toward the
food to
be frozen 114, which also is disposed in a suitable food rack 119. The motors
for the
fans 116 are sealed in the wall to reduce heat transfer from the motors to the
interior
of the freezer 206. A C02 gas nozzle 112a is provided near the food rack 119
which
supplies variable cooling when necessary. The control unit 104 is coupled to
the
fans 116, C02 source 112b, and a thermocouple (as illustrated in Fig. 3)
inserted into
an item of food (e.g., sushi). The control unit is configured to control the
fans 116
and C02 source 112b to adjust the level of cooling energy depending upon the
temperature of the food, and to cool the food as defined by the present
invention.
Freezer 206 may be used as the second, interior freezer in the dual freezer
embodiment in Fig. 1A or may be used as the single freezer in a single freezer
configuration.of the present invention
[0037] The size of freezer in accordance with the present invention can be of
any
suitable size depending on the quantity of food to be frozen. In one
embodiment,
freezer 206 is approximately8' X 8' X 8' and can be used to freeze
approximately
two to three 200 pound batches of sushi according to the present invention. In
this
embodiment, approximately 400 pounds of dry ice is placed in racks 108. Also,
the
freezers are preferably capable of maintaining a positive air pressure inside
of
approximately 5 psi to maintain the dry ice and to allow the dry ice to
sublimate
properly for the desired cooling. To maintain the pressure, a pressure relief
valve
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(not shown) may be provided to vent the freezer when necessary if the pressure
is
increasing.
[0038] The temperature sensors 118 may also be placed in the vicinity of the
food
114 or any other location within freezers 106 and 206 to allow proper
monitoring
thereof. For example, as shown in Fig. 3, a temperature sensor 118a may be
mounted in the interior of freezers 106 and 206 to measure the temperature of
the
freezer environment. Fig. 1 illustrates an example of mounting the temperature
sensor 118a in the interior of freezer 106. Also, as shown in Fig. 3, a
temperature
sensor 118b is preferably connected inside the food product 114 to monitor the
interior temperature of that food product. Temperature sensors 118a and 118b
are
preferably connected to the control unit 104 so that the interior and core
food
temperatures can be monitored and controlled. As illustrated in Fig. 3, the
temperatures are preferably displayed on a monitor.
[0039] In another embodiment of the present invention, a temperature sensor is
positioned to measure the surface temperature food product. The surface
temperature of the food produce, which largely corresponds to the temperature
of
the interior of the freezer, may be used to provide additional information for
freezing
food producfis in accordance with the present invention.
[0040] The control unit 104 is configured to control the speed of the
circulation of
air over the dry ice. Also, control unit 104 may control the interior
temperature of fihe
freezer 106, including the variable cooling source 112a and 112b as needed to
ensure that the food 114 is cooled at the proper rate. For example, if the
temperature of food to be frozen is not decreasing at the desired rate, the
variable
cooling may be initiated to further reduce the temperature inside second
freezer 106
or freezer 206 at the desired rate. The control unit 104 also may reduce or
terminate
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the variable cooling to prevent the outside region of the food from cooling
too quickly
so that the food is frozen throughout its bulk properly. For example, carbon
dioxide
gas may be discharged into second freezer 106 or freezer 206 via nozzle 112a
for a
predetermined amount of time (e.g., a few seconds), or until the environment
or food
(surFace and/or core) reaches a selected temperature.
[0041] In another embodiment of the present invention, the freezer system may
be configured for continuous high volume operation by providing conveyor
mechanism or the like for loading and unloading units of food to be frozen.
One
example of a continuously operating freezer 300 is shown in Figs. 2A-2B.
[0042] Fig. 2A is a side view and Fig. 2B is a top view of an exemplary
"tunnel"
style freezer 300 according to one embodiment of the present invention. In the
tunnel style freezer 300, a conveyor belt assembly 130, which may include one
or
more conveyor belts, can be provided for continuous delivery of foods to be
frozen.
To accommodafie the conveyor belt 130, a load lock means 132 may be included
to
maintain temperature inside the freezer 106 and prevent loss of cooling energy
during loading and unloading. For example, the conveyor belt assembly 130
preferably includes three conveyor belt sections 130a, 130b and 130c, one on
each
side of the freezer 106 and one inside freezer 106 as illustrated in Fig. 2C.
Each
load lock means 132 may include two doors 132a, an exterior door
(loading/unloading gates) and an interior door (loading/unloading lock gates),
and a
loading/unloading section or housing 132b. The doors 132a may open and close
rapidly to allow batches to enter and exit the freezer 106 and can be
configured to
prevent loss of cooling energy to the freezer 106. For example, the exterior
doors
132a may not open unless the interior doors 132a are shut, and vice versa.
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[0043] Referring to Fig. 2B, the conveyor belt may pass between the dry ice
racks
108, and the rest of the freezer 106 configuration may remain the same as the
embodiments already described above in connection with Figs. 1A-1 B. In this
configuration, temperature sensors may be permanently disposed within the
interior
of freezer 106, or wireless sensors are contemplated that could be inserted
into the
food before freezing and removed thereafter.
[0044], In a preferred embodiment, the food products to be frozen, such as
sushi,
should first be packaged into a container, such as a bag, and hermetically
sealed
after de-aeration. Such packaging locks flavor into the product and helps
prevent
the food from drying. Shrink wrapping or vacuum bagging the food allows good
results and is preferred.
[0045] Operational aspects of the present invention are discussed in
connection
with a discussion of the temperature characteristics of the environment of the
interior
of the second freezer 106 and of the food during freezing. For example, in .an
experiment, an arbitrary volume of cooked rice (2 Ibs) was cooled to room
temperature (about 22°C) and stored in a bag after it was determined to
be in a
balanced condition. The package was de-aerated and sealed. The food was then
stored in the interior of freezer 106 maintained at a temperature of -60 to -
70°C.
Temperature sensors were used to measure (1 ) the environment or reference
temperature of the interior space of second freezer 106, and (2) the core
temperature of the food 114.
[0046] The results of the experiment are shown in Fig. 4 Curve A is the
cooling
transmission rate curve of the temperature inside the interior of freezer 106.
Curve B
shows the temperature of the core of the food to be frozen.
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[0047] Curve A reflects the measured interior environment temperature of
freezer
106, which also reflects the cooling capacity of the freezer. The interior
environment
temperature A of the freezer changes as a function of time because of thermal
energy exchange using the air in the freezer as a catalyst. In other words,
the
environment temperature A shows the transition in the freezer caused by
thermal
transmission from the outside surFace of food product, such as a rice cluster,
which is
warmer than the environment temperature, as air passes over the food product.
This
temperature inclination changes the degree of the angle by the freezer
capability per
unit of a chiller source, wind velocity and size of transfer surFace area,
etc., however
it can be read that the change of inclination has a general tendency which is
affected
by the thermal capacity of the rice cluster.
[0048] Cooling control of the freezer can be determined from the curves, such
as
the curves represented in Fig. 4. When curve B reaches the maximum ice crystal
generation zone, it can be observed that the angle of curve A begins to
flatten, which
indicates the lack of heat transmission energy of the freezer 106. If this is
detected,
a cooling control will be applied to increase transmission energy.
[0049] The freezing activity is achieved by seeking the phase inversion, by
passing the temperature of the food through its freezing point artificially. A
complex
group of solid-state properties has many different freezing points, especially
food
which is a complex of hydrous substances, like sushi, the ingredients of which
may
have significantly different water characteristics to be carefully treated.
Since curve
A is the curve of the controllable buffer zone in a cooling process, it shall
be
considered as a control region such that the cooling heat energy, the
transmission
speed for the heat exchange, etc. and cooling transmission temperature control
should be applied within this zone.
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[0050] Curve B is considered as the cooling heat conduction area of the rice
cluster by which the cooling heat transmission is undertaken, and it should be
understood as an analytical area for a proper control of the hydrous
properties of the
food. That is, from curve B, it can be determined how to adjust the cooling
within the
freezer 106, as more or less energy is required to achieve the desired cooling
of the
food.
[0051] It can be observed that curve B has a shallow angle as the temperature
goes below 0°C and continues until a point where curve B reaches
approximately
-10°C. From this observation, it can be understood that the heat
conduction ratio of
the food reduces following the progression of ice precipitation in the food
between
the surface and the core of the food due to ice precipitation of menstruum
(free
water) at the surface of a rice cluster. Also, each rice grain is individually
affected by
the changes in the thermal conductivity from the outside to the core of the
rice
cluster, and it is therefore understood that curve B reflects the heat
exchange rate of
the area between the surface and the core of the food as the aggregate of
average
complicated heat flow speed.
[0052] Curve B also shows the similar tendency as curve A. However, while
curve A corresponds to a transmission rate with comparatively high efficiency
by the
direct heat dissipation transfer to the environment temperature, curve B shows
a
widening temperature difference from curve A by relaying to the layer where
the
conduction efficiency is low in the progression of heat flux process from the
curve A,
and in spite of the rapid declining angle of curve A, continues as being
indicated an
aspect of passing through a temperature zone of the specific food. Meanwhile,
each
layer from the exterior side to the core side of the. food advances mainly the
phase
changes of free water and relay descent in the direction where the constituent
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frozen, and the temperature thereof passes through the maximum ice crystal
generation zone.
(0053] At this stage, curve B shifts to the steep angle. The difference
between the
temperatures of core side and the exterior side becomes narrow and finally,
overlap
each other, and the thermal conductivity of the each layer of rice cluster
become
almost equivalent, and the freezing is deepened in proportion to the heat
transmission capability from this point. This indicates that all the food
throughout its
bulk has been cooled passed the maximum ice crystal generation zone.
[0054] From Fig. 4, the relationship between the interior environment
temperature
of the freezer 106 and the surface and the core temperatures of the food being
,
frozen, can easily be estimated. Additionally, the amount of conduction
between the
surface of the food and the core can also be calculated. Accordingly, the
present
invention can be configured to estimate the temperature of the food from the
measured interior environment temperature, in lieu of measuring the
temperature of
the food directly. For example, control unit 104 may be programmed with an
algorithm for calculating estimated surface and core temperatures of the food
from
the interior temperature of the freezer based on, for example, the curves of
Fig. 4.
From these estimated temperatures, the control unit 104 can control the
variable
cooling 112, adjustment unit 110 and fans 116 to cool the food at the proper
rate.
[0055] A dotted line curve in Fig. 4 shows an example of the temperature drop
when variable cooling in the form of carbon dioxide gas is injected into the
interior of
the freezer.
[0056 Fig. 5 is a flow diagram of a method for freezing food according to an
embodiment of the present invention. First, at step 5-1, the food to be frozen
is
packaged. In a preferred embodiment, the food is de-aerated and vacuum bagged,
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shrink wrapped or the like, when the food reaches room temperature or
approximately 22°C. Then at step 5-2, the food is placed in the freezer
to begin the
freezing process. In a preferred embodiment, the food is at room temperature,
approximately 22°C, when placed in the freezer. In an alternative
embodiment, the
food is placed in the freezer at a temperature at which it is cooked (i.e. 60-
80°C).
For sushi, the food is frozen preferably within 1-2 hours after the rice is
cooked. The
food to be frozen can be packed as described above and placed in the freezer
106 of
systems 100-300 to begin the freezing process.
[0057] At step 5-3, the temperature inside the freezer 106 is measured via
temperature sensors 118. As described above, the temperature of the food
(surface
and/or core) may be estimated using a temperature inclination of the
atmospheric
temperature from the chart of Fig. 4. Alternatively, temperature sensors 118
may be
used to measure the temperature of the food directly.
[0058) When the temperature of the food 114 reaches the upper limit of the
accelerated aging temperature zone (e.g., for sushi, approximately 10°
C), a cooling
pattern is generated to cool the food through the accelerated aging
temperature
zone. For example, the control unit 104 controls the adjustment unit 110 and
the
fans 116 to create an operative cooling pattern (i.e., the fans blow air over
the dry
ice). Control unit 104 may also initiate variable cooling via variable cooling
units 112,
if cooling is too slow. Variable cooling injection then can be combined with
circulation control by the control unit 104, and the temperature of the food
is
decreased through the accelerated aging zone at the appropriate rate.
Preferably,
the temperature of the food is reduced quickly to properly freeze the food
throughout
its bulk without damage to the food cells. Preferably, the accelerated aging
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temperature zone (approximately 6°C to about 0°C) is traversed
in 1-10 minutes, and
preferably 3-5 minutes.
[0059] At step 5-4, when temperature of the surface of the food reaches the
upper limit of the ice crystal generation zone (e.g., for sushi ~ 0°C),
variable cooling
is adjusted again, if necessary, in response to heat transmission of the food.
Variable cooling may be terminated if the temperature of interior freezer 106
is
sufFicient to continue cooling of the food through the ice crystal generation
zone at an
adequate rate and to prevent the food from cooling too quickly. Variable
cooling may
not be necessary to freeze food at the proper rate. If the temperature of the
food
does not reach approximately -5°C to approximately -7°C within
approximately 10-15
minutes after the food is introduced into the freezer, variable cooling may be
initiated
to force the temperature to go down momentarily as shown with the dotted line
of
curve A in Fig. 4, as an example to assure that the temperature of the food
decreases to the desired range. One skilled in the art will understand that
cooling
may necessarily require adjusting based on factors such as the size of the
freezer,
the amount of food to be frozen at a time, etc.
[0060] The food is cooled from 0°C to -10°C in approximately 10
to approximately
40 minutes. The food is preferably cooled from 0°C to -10°C in
approximately 15 to
approximately 30 minutes. In another preferred embodiment, the food is cooled
from
0°C to -7°C in approximately 10 to approximately 40 minutes.
[0061] Next, the food is preferably cooled from about -10°C to about -
30°C within
approximately 30 minutes to approximately 90 minutes. The food is more
preferably
cooled from about -10°C to about -30°C within approximately 40
to 60 minutes. By
the time the food reaches -30°C, the fans will most likely become
unnecessary and
may be shut off. At this temperature, the water inside the food is frozen
completely.
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[0062] Next, the food is cooled to about -60°C, in order to freeze
composite water
that may exist, such as water mixed with oil. Preferably, the food is cooled
to -60°C
in approximately 5 to approximately 50 additional minutes. More preferably,
'the food
is cooled to -60°C in approximately 10 to approximately 30 additional
minutes. At this
point, the food is completely frozen throughout.
[0063] The velocity of coolant circulafied in the freezer, such as by a fan,
is
preferably set to be proportional to the heat transmission efficiency. It is
considered
that the stronger the velocity of the coolant, the better the heat exchange
rate is.
However, the velocity of the coolant in the freezer shall be controlled in
consideration
of the whirlpool motion of air circulating therein and the proper heat
exchange in the
relation between the flow and the obstruction.
[0064] As for the variable cooling, liquid nitrogen and a liquid carbon
dioxide can
be considered as a coolant. From the aspect of the evaporation temperature and
the
evaporation latent heat, the nitrogen has -196°C/47Kcal and carbon
dioxide has
-78.9°C/137 Kcal. A coolant which has more evaporation latent heat
within the
range of -60°C is most suitable. Carbon dioxide gas is preferred.
[0065] Temperatures and times described herein are described in connection
with
preferred embodiments. One skilled in the art will understand that the
temperatures
and times may differ based on the composition of the food, the size and type
of the
freezer, etc.
[0066] (n accordance with another aspect of the present invention, a system
and
method for thawing frozen food is described with reference to Figs. 6, 7A and
7B.
When thawing a container of vacuum packed frozen food 202, such as sushi, a
container of a solution or gel 204 is placed on top of package of the food. In
the
case of sushi, container 204 is placed on the side of the sushi topping.
Preferably,
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the container 204 is flexible, like a bag, to allow good surface contact with
the food
202. The cooling solution in the bag 204 should preferably fit any contour of
the
frozen food container 202 (water, gel, jelly, etc.).
[0067] As illustrated in Fig. 6, the food can be thawed conventionally with a
steamer, with the heating energy applied to the bottom of the frozen food
container.
The cooling solution 204 on top of the food 202 allows, in the case of sushi,
the rice
portion to be defrosted to a slightly warm condition while topping (raw fish,
etc.) is
maintained in chilled condition by the cooling solution on top. Thus, the
present
invention provides a very inexpensive method for defrosting food that can be
performed by anyone and at any volume.
[0068] Another embodiment of the present invention is shown in Figs. 8A-8C.
System 700 is a warm water thawing system that includes a tray 705 and a water
source 702. The tray 705 may be disposed at an angle to allow gravity assist
with
water flow. The tray has three sides or lips 707-709 and a fourth side 706 is
left open
to allow the water to drain from the tray. As illustrated in Fig. 8A, frozen
food 202 is
preferably arranged in the tray such that water from source 702 flows under
and
along the sides of the food 202.
[0069] Similar to the method described with reference to Fig. 6, a cooling
pack
204 is preferably placed on top of the frozen food containers 202. For sushi,
this
keeps the topping cool while the rice side is warmed by the water. The water
may
be at any appropriate temperature to thaw the food at the desired rate such
as, for
example, approximately 60°C to 90°C and preferably 60°C
to 80°C. The water level
is preferably controlled so that the warm water does not reach the topping
side of the
sushi. The food is preferably thawed in approximately 5 to 45 minutes, more
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preferably thawed in approximately 10 to 20 minutes, and most preferably
thawed in
approximately 10 to 15 minutes.
(0070] With the system 700, a large volume of frozen food may be thawed at the
same time.
[0071] Fig. 9 illustrates another system for thawing food products in
accordance
with another embodiment of the present invention. In particular, Fig. 9
discloses a
device 900 for containing a medium 903 for thawing the food product, such as
sushi.
The device 900 can be any suitable device for containing the medium 903, such
as a
container or tray. In a preferred embodiment, the device 900 includes a means
for
heating the contents of the device. The means for heating can be any suitable
means for heating the contents of the device such as an electrical heating
element
904. Electrical heating element 904 can be connected to any suitable power
source,
such an electrical outlet, via plug 902. In a preferred embodiment, the medium
903
is water. Medium 903 could also be any suitable heat conducting medium.
[0072] As illustrated in Fig. 9, the food product 202 is placed in the device
900
with the cooling pack 204 preferably placed on top of the food 202. Medium
903,
such as water, is also placed in the device 900 and is heated to a temperature
which
is desired to thaw the food product 202. In a preferred embodiment, the level
of
medium 903 in the device 900 is controlled so that it does not reach the
topping side
of the food product 202, such as sushi. Also in accordance with a preferred
embodiment, a temperature sensor 901 can be used to monitor and control the
temperature of the medium 903 in the device 900.
[0073] Thus, the invention has been described in connection with what are
presently considered to be the most practical and preferred embodiments. It is
to be
understood that the invention is not to be limited to the disclosed
embodiments, but,
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on the contrary, is intended to cover various modifications and equivalent
arrangements included within the spirit and scope of the appended claims.
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